1. Field of the Invention
This invention relates to digital telephone communication systems (employing pulse-code modulation [PCM]) with redundancy reduction; more particularly, the invention concerns a system for reducing the redundancy of pulse-mode modulation encoded speech signals by means of variable companding laws and a variable number of bits per sample.
2. Description of the Prior Art
The majority of pulse-code modulation transmission systems presently being manufactured provide for time-division multiplexing and analogue-to-digital conversion of a given number of speech channels, such as generally 30 such channels. These prior art systems are characterized by an overall transmission rate of about two million bits per second, with each speech channel being sampled every 125 .mu.sec and a sample being encoded in eight bits. Consequently, a transmission rate of 64,000 bits per second corresponds to each such speech channel.
Prior art pulse-code modulation systems employ logarithmic companding in order to make the signal-to-noise ratio as constant as possible over a wide volume range of the input signal. The primary reason for this is that in such prior art systems, the companding law is fixed, i.e., independent of the structure and form of the input signal, and, therefore, an attempt to cover the volume range needed for all kinds of talkers (e.g., +6db to -50db) in the majority of pulse-code modulation systems, especially in Europe, a logarithmic companding law (such as one approximated by 13 linear segments) associated with 8 bit sample encoding has been utilized. Such prior art logarithmic companding associated with sample encoding results in a constant, precision measurement since the region of small amplitude signals is finely subdivided whereas the region of large amplitude signals is only roughly subdivided. Thus, if the speech signal stays at a high level for a relatively long time, the fine subdivision which occurs due to the fixed companding law in the low level region is useless; while if the speech signal stays at a low level for a relatively long time, the rough subdivision which occurs due to the fixed companding law in the high level region is similarly useless. Thus, the aforementioned prior art conventional systems are not satisfactory in that the use of a fixed companding law results in the necessity of using a high number of bits per sample. These disadvantages of the prior art are overcome by the present invention.